The invention relates to a temperature sensor for a sheet-shaped heating element, and more particularly, to a temperature sensor which may be used, for example, in an electrophotographic copying machine including a heater to fix a toner image transferred onto a record paper by heating it and in which the operating temperature of a sheet-shaped heating element used in the heater is determined by means of a thermistor.
In determining the temperature of a sheet-shaped heating element which is used in a heat fixing unit for an electrophotographic copying machine, a thermistor may be used as illustrated in FIG. 1. As is well recognized, a thermistor 1 comprises a heat sensitive element 1b which is disposed within an envelope 1a, formed of a material such as glass or heat resistant plastic, toward the lower end thereof, and a pair of lead wires 3 having their one end connected to the element 1b at spaced points and having their other end taken out of the envelope through the top end thereof. When used as a temperature sensor, the thermistor 1 is disposed so that its lower end is in contact with the surface of a sheet-shaped heating element 2, as illustrated. However, when the thermistor 1 is disposed in this manner, it is difficult to provide an accurate determination of the temperature of the heating element 2 since a mere contact of the free end of the thermistor with the sheet-shaped heating element 2 is unstable and causes a variation in the spacing between the heat sensitive element 1b and the heating element 2. In extreme cases, the heat sensitive element 1b may be spaced from the heating element 2 due to oscillations of a heater, making it impossible to provide an arcuate determination and control of the temperature.
To avoid the above difficulty, FIG. 2 illustrates thermistor 1 which has an ellipsoidal configuration so that it may be disposed in lengthwise contact with the sheet-shaped heating element 2, as shown. While this arrangement provides a more arcuate determination of the temperature as compared with the arrangement of FIG. 1 because the heat sensitive element 1b is located close to the heating element 2, it still suffers from the disadvantages that the thermistor may move away from the heating element due to oscillations of the heater. In addition, because the lead wires 3 are located close to the heating element 2, there occurs a problem of leakage between the lead wires, causing a likelihood that an error in the measurement may be produced.